The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Hydraulic fracturing is a process whereby a subterranean hydrocarbon reservoir is stimulated to induce a highly conductive path to the formation, increasing the flow of hydrocarbons from the reservoir. A fracturing fluid is pumped at high pressure to crack the formation, creating larger passageways for hydrocarbon flow. The fracturing fluid may include a proppant, such as sand or other solids that fill the cracks in the formation, so that, when the fracturing treatment is done and the high pressure is released, the fracture remains open.
Key to a successful fracturing operation is the accurate monitoring of the bottom hole pressure in the wellbore, and determining when to stop pumping fracturing fluid and initiate flush of the wellbore. Early initiation of the flush results in less than optimal fracturing of the hydrocarbon bearing formation and a less productive well. However, surface pressure measurements are prone to result in just such early initiation of the flush. This is because the pressure at the surface does not accurately reflect the conditions at the bottom of the wellbore. In particular, surface measurements include additional effects such as the friction of the flowing slurry along the length of the wellbore or the constantly changing hydrostatic pressure of the proppant laden fracturing fluid. Modeling these effects is typically not accurate enough to determine precisely when to initiate the flush based upon the surface pressure. On the other hand, if the flush is initiated too late, the pumping of additional slurry leads to wellbore screen-out, where the proppant backs up into, and fills the wellbore.
Wellbore screen-out is undesirable because the proppant restricts the free flow of hydrocarbons in the wellbore and, in the extreme, can trap downhole assemblies in the wellbore. If the wellbore screen-out is significant enough, the entire process of perforation and fracturing must be stopped while wellbore repair is performed. During repair, the overpressure is released, permitting ball sealers, put in place after previous fracture treatments, to fall out, and precluding further fracturing after the repair is completed, without the placement of additional wellbore plugs. Therefore, repair of a wellbore after a wellbore screen-out is expensive and time consuming.
From the foregoing it will be apparent that there remains a need to measure bottom hole pressure during fracturing operations to accurately detect tip screen-out and prevent wellbore screen-out.